1. Field of Invention
The invention relates to an image display system and a manufacturing method, and, in particular, to an image display system with a multi-gate TFT and a method of manufacturing the multi-gate TFT.
2. Related Art
Active matrix display type image display systems include an active matrix liquid crystal display (LCD) device and an active matrix organic light emitting diode display device, for example. A thin-film transistor (TFT) is disposed in a pixel of the active matrix display device, so the number of color levels and the frame holding ability, which may be controlled in the active matrix display device, are higher than those of a passive matrix display type image display system, and the better display effect can be provided.
The element property of the TFT influences the display quality of the image frame. The prior art utilizes an excimer laser to anneal a silicon film layer of the TFT so that the silicon film layer is recrystallized into polysilicon. Thus, the polysilicon TFT has the higher electron mobility, the lower threshold voltage, the stabler saturation property and the lower power consumption, and can integrate a pixel array and a driving circuit on the same glass substrate, or even integrate the whole image display system on the same glass substrate.
FIG. 1 shows a relationship between a drain-source voltage and a drain-source current in a conventional TFT. As shown in FIG. 1, the TFT structure is a floating body structure. So, when the voltage difference between the drain and source is too high, the TFT is influenced by the kink current and thus has the poor stability. The kink current is generated due to the increase of the potential energy of the body caused by the accumulated holes in the source. Thus, the behavior between the body and the source is similar to a parasitic bipolar junction transistor (BJT). If the electric field of the body becomes stronger, the turn-on current (i.e., the kink current) becomes higher.
FIG. 2 is a schematic illustration showing a conventional dual-gate TFT. As shown in FIG. 2, the related art utilizes a dual-gate structure to case the influence caused by the too-high voltage difference between the drain and source and to lower the kink current. A conventional TFT 1 is configured to be composed of two sub-TFTs T1 and T2. The TFT 1 includes a silicon film layer 10 and a metal layer 16, which serves as a gate of the TFT I. The gate includes two interdigitated gate electrodes 161 and 162 respectively serving as gates of the sub-TFTs T1 and T2. The silicon film layer 10 is doped to form three doped regions 11, 12 and 13. The doped regions 11 and 13 respectively serve as the source and the drain of the sub-TFT T1, while the doped regions 12 and 13 respectively serve as the drain and source of the sub-TFT T2. Taking the TFT 1 as a whole, the doped regions 11 and 12 respectively serve as the source and drain of the TFT 1. In addition, channel regions 14 and 15 of the sub-TFTs T1 and T2 are formed corresponding to the gate electrodes 161 and 162 in the silicon film layer 10. Under this architecture, the potential difference between the drain and source in each of the sub-TFTs T1 and T2 will be lower than that of the original single-gate TFT. Thus, the kink current of each of the sub-TFTs T1 and T2 is thus lower, so the kink current of the overall TFT 1 is also improved. As for a high-efficiency image display system, however, the efficiency of the image display system can be further enhanced if the kink current of the TFT can be further lowered.
Therefore, it is an important subject to provide an image display system and a method of manufacturing a multi-gate TFT.